Preparation method for ultra low ash coal-water slurry

ABSTRACT

A method for preparing ultra-low ash coal-water slurry includes: (A) grinding of coal to a size below the upper size limit and then making a proper concentration slurry; (B) screening coal by using the slurry particle separation device and separating them by density into tailings, middlings and clean coal, adjusting the fraction of clean coal production according to the ash content standard of final product and then producing the clean coal slurry and tailings slurry; (C) wet grinding of the middlings in step B; (D) reprocessing the product in step C by step B. The upper size limit can be 0.2 mm, 0.01 mm or 0.001 mm, and the ash content can be below 2%, 1% or 0.5%. This method can largely enhance the production rate of clean coal, improve the coal-water slurry fuel&#39;s particle size graduation, improve the fluidity and increase the particle concentration of the coal-water slurry fuel.

TECHNICAL FIELD

The present invention relates to a method for preparation of ultra-low ash coal-water slurry.

BACKGROUND ART

Current coal-water slurry technology, which has enabled the heavy oil replacement in boilers, has successfully solved the coal particle grinding-slurry making problem and been widely used in industry. To replace more oil products by coal-based fuel, such as using coal-water slurry in non-retrofit oil fired boilers, internal combustion engines or gas turbines, some work needs to be done to reduce the ash content in coal especially those inorganic particles which can cause equipment erosion. Current method for preparation of clean coal is shown in the patent CN97116584 issued on Jan. 8, 2003. In this method, emulsifier is added into the coal-water slurry with micro-scale particles. Hydrophobic coal particles are aggregated while hydrophilic mineral particles aren't. Ultra-low ash clean coal is then produced in the froth flotation process. The drawback of this method is that it is ineffective for coal with high pyrite content and more emulsifier agent needs to be consumed. So the ash content of final product is not easy to control but depends on coal properties.

SUMMARY OF THE INVENTION

The present invention introduces a method for preparation of ultra-low ash coal-water slurry. This method does not consume emulsifier agent and is applicable to coal with high pyrite content.

This method comprises the following steps:

Method 1

A. wet or dry grinding of coal to a size below the upper size limit and then making a proper concentration slurry;

B. screening coal made in step A by using slurry particle separation device and separating them by density into tailings, middlings and clean coal, adjusting the fraction of clean coal production according to the ash content standard of final product and then producing the clean coal slurry and tailings slurry;

C. wet grinding of middlings in step B;

D. reprocessing the product from step C by method in step B.

If wet grinding is used in step A of the above method, steps can be adjusted to be:

Method 2

A. wet grinding of coal to a size below the upper size limit and then making a proper concentration slurry;

B. screening coal made in step a by using slurry particle separation device and separating them by density into tailings, middlings and clean coal, adjusting the fraction of clean coal production according to the ash content standard of final product and then producing the clean coal slurry and tailings slurry;

C. reprocessing the middlings in step B by method in step A.

The theoretical basis of this method is described as follows. Most of the mineral content of coal is mineral particles which exist independently or are combined with organic in coal. Those independently existing mineral particles or aggregates with low organic content have higher density than pure coal and will be separated as gangue in regular coal screening process. Aggregates with high organic content have similar density as pure coal particles. So it is difficult to separate them in the jig screening process. Organic and mineral particles can be separated by cyclic grinding and chosen as clean coal or tailings in slurry particle separation process. Few inorganic mineral contents of final product include non-particle shape minerals within coal particles, ultra-fine mineral particles dissipated within coal particles, and ultra-fine particles attached to coal particles or inseparable tiny mineral particles.

This method can largely enhance the production rate of clean coal, improve the coal-water slurry fuel's particle size graduation, improve the fluidity and increase the particle concentration of the coal-water slurry fuel.

Centrifugal jig screening is the key in these steps. According to the Stokes's law, the settling velocity of coal particles in the slurry is very low under normal gravity when coal particles are grinded to a very small size. Particle settling may even stop due to particles interaction force and disturbance caused by ultra-fine particles. Therefore, regular screening jig can't be used for screening small particles under this circumstance. The principle of centrifugal jig is as follows. Regular screening jig is put under a field with tens of gravity caused by centrifugal and gravitational combined forces. Small coal particles can settle quickly in the slurry and fine coal slurry can be screened. More details of this centrifugal jig can be found in Chinese patent CN201010123867.8.

When the slurry contains particles with larger upper size limit and average size, for example, 0.2 mm upper size limit and average particle size of 0.075 mm, particles can be separated by other equipments, such as Falcon C series device made by Canada Sepro Systems International. This separator can only separate and get two final products. To separate coal into clean coal, middlings and tailings, this separator needs to be used in two steps.

Tailings said in the method is a high density screening product with high inorganic mineral content, also called gangue.

Final product said in the method is the clean coal slurry obtained after completing all steps in this method.

Wet grinding said in the method can be done by using commercial ball mills, rod mills, stirred mills, or water jet technique. Chinese Patent CN201010123854.0 introduces a slurry particle grinding apparatus utilizing water jet and stirred grinding. This apparatus is especially applicable for wet grinding in this invention.

Dry grinding said in the method can be done by using commercial dry ball mills, column mills, or the apparatus for the dry grinding of solids disclosed in U.S. Pat. No. 5,695,130 issued on Nov. 9, 1997.

Proper concentration said in the method is the concentration which enables the slurry transferable by using pumps. It can be higher or lower than the concentration of final product.

Upper size limit said in the method depends on the application of final product. For application in the replacement of heavy oil in boilers, the upper size limit can be 0.2 mm. For application in the replacement of diesel fuels, the upper size limit may be below 0.01 mm, or even below 0.001 mm.

The ash content of final product also depends on the application. For production of carburant or activated carbon, the ash content should be no higher than 2% or 1%. For replacement of diesel fuels, the ash content should be no higher than 0.5% or less. The ash content is the percentage of ash measured by slow ash method in dry coal.

Adjusting the fraction of clean coal production according to the ash content standard of final product refers to adjusting the clean coal production in the slurry particle separation process according to the properties of raw coal and the ash content standard of final product. If the ash content of raw coal is low and final product can have high ash content, then the fraction of clean coal production will be high. If the ash content of raw coal is high and final product should have low ash content, then the fraction of clean coal production will be low.

This method has no special requirements for the type and origin of raw coal. But low ash coal especially those with low clay minerals or anthracite is preferred. Clay minerals tend to become ultra fine particles during the final screening process and aggregate with coal particles. This could increase the ash content of final product.

Raw coal's initial screening, initial grinding and those unrelated to ash removal process such as dehydrating, slurry concentration adjusting or stabilizer adding are not included in this invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of preparation of ultra-low ash coal using dry fine grinding as specified in the first embodiment according to the present invention;

FIG. 2 is a schematic of preparation of ultra-low ash coal using wet fine grinding as specified in the second embodiment according to the present invention;

FIG. 3 is a cross-section view of the centrifugal jig;

FIG. 4 is a schematic of the tilt jig component and parabolic shape sieve;

FIG. 5 is a 3D schematic of the main body of slurry particle separation device;

FIG. 6 is a top perspective view of the main body of slurry particle separation device;

FIG. 7 is a front perspective view of the main body of slurry particle separation device;

FIG. 8 is a 3D cross-section view of the main body of slurry particle separation device, cut along line A-A1-A′1-A′ as shown in FIG. 6 with its front portion removed;

FIG. 9 is a 3D cross-section view of the slurry particle separation device with protruding fins, cut along line A-A1-A′1-A′ as shown in FIG. 6 with its front portion removed; and

FIG. 10 is a 3D cross-section view of the slurry particle separation device with protruding fins, cut along line B-B′ as shown in FIG. 7 with its upper portion removed.

PREFERABLE MODE OF CARRYING OUT THE INVENTION Embodiment 1

As shown in FIG. 1, fine coal is dumped into coal pit 100. Skip hoist 200 lifts coal from coal pit 100 and sends it into column mill 300. Column mill 3 grinds coal into particles with size of −0.1 mm. Belt conveyor 400 transfers coal particles into agitator 500 in which water is added through water inlet 1100 until the coal concentration is around 65%. Pump 600 transfers the coal-water slurry into centrifugal jig 700. The ash content of final product is controlled below 2% by adjusting the production rate of clean coal. Clean coal slurry is pumped into clean coal slurry tank 800 or into post process. Tailings slurry is pumped into tailings slurry tank 900 or into dehydrating process. Middlings slurry is pumped into ball mill 1000 for wet grinding and then is pumped back into agitator 500.

Embodiment 2

As shown in FIG. 2, fine coal is dumped into coal pit 100. Skip hoist 200 lifts coal from coal pit 100 and sends it into roll crusher 1200. Roll crusher 1200 crushes coal into particles with size of −0.5 mm. Belt conveyor 400 transfers coal particles into agitator 500 in which water is added through water inlet 1100 until the coal concentration is around 65%. Reciprocating pump 1300 increases the pressure of the coal-water slurry to 10-40 Mpa and transfers the mixture into slurry particle grinding apparatus 1400. Slurry of particles with size of −0.01 mm is pumped into centrifugal jig 700. Slurry of particles with size of +0.01 mm is pumped back into agitator 500. The ash content of final product is controlled below 0.5% by adjusting the production rate of clean coal. Clean coal slurry is pumped into clean coal slurry tank 800 or into post process. Tailings slurry is pumped into tailings slurry tank 900 or into dehydrating process. Middlings slurry is pumped back into agitator 500.

Centrifugal jig 700 and slurry particle grinding apparatus 1400 are detailed as follows. They have the same structure as those disclosed in Chinese Patents CN201010123867.8 and CN201010123854.0 published earlier than the present application.

As shown in FIG. 3-FIG. 4, the centrifugal jig 700 comprises a rotating device 1, at least one jig unit 2 and a product receiver 3. The rotating device 1 includes a rotor 11 and base 12. The rotor 11 driven by a power drive unit 121 rotates steadily around the axis 1111 of a vertical shaft 111. The jig unit 2 includes a hutch chamber 21, a screen residue chamber 22, a screen 23, a feeding device 24, a discharging device 25 and a pulsator 26. With similar structure and function as the conventional jig, the jig unit is used to separate particles of different density in the slurry. The jig unit 2 is installed on the rotating device 1 at an inclined angle α along the movement of the feed. The hutch chamber 21 is located far away from the vertical shaft 111. The inclined angle α is equal to the angle between the vector sum Fr of the gravitational force G and centripetal force Fc acting on the jig unit 2 and vertical direction. Since the centripetal acceleration can reach tens of gravity, this angle is close to 90 degrees. The jig unit 2 operates under the resultant force field generated by the gravitational force and centrifugal force due to rotation. The product receiver 3 is an annular tank fixed on the ground to collect the separated materials discharged from the rotating jig unit 2. The discharge device 25 of the jig unit 2 discharges materials through outlets instead of through the screen. The outlets include a heavy material outlet 251 and a lightweight material outlet 252. The screen residue chamber 22 which is open in this example is also called launder. During the jigging process, small amount of particles S go through the screen 23 into the hutch chamber 21. Then they are discharged at the bed material outlet 221 on the bottom of the hutch chamber and goes into the product receiver 3. The screen 23 can be a screen with mesh size of 0.2 mm and feldspar ragging bed. The suction force is controlled during machine operation.

Feed slurry S is accepted on the top of the vertical shaft 111 through a feed inlet 240 into a conical shape tub 241 which rotates with the jig unit simultaneously. The slurry then goes through a feed pipe 242 into the hutch chamber under the screen 23. Due to the centrifugal force and push by successive feed, the feed slurry S stratifies and moves upwardly. The lightweight slurry SL and heavy slurry SH discharge at the lightweight slurry exit 251 and heavy slurry exit 252, respectively. Then they enter the corresponding lightweight slurry receiver 31 and heavy slurry receiver 32. The pulsator 26 uses a conventional diaphragm pulsation device 261. As the diaphragm 2613 moves due to the action of the cam drive shaft 2612 powered by the pulsation motor 2611, pulsation is generated in the hutch water. To keep steady rotation of the rotating device 1, a heavy balance object 112 is put on the opposite of the jig unit.

Ideally, the screen 23 has a curved surface formed by revolving a parabola 231 which extends along the movement of the feed around the axis 1111 of the vertical shaft 111. In this way, the screen 23 and the jigging fluid can share the same surface. Based on this, the inclined angle of the screen 23 can be adjusted according to the refuse discharge method. If the screen 23 is narrow and centrifugal force is very large, flat screen plate can be used.

Power supply of the rotating jig unit 2 can be maintained by connecting the slip ring of the vertical shaft with power source of the fixed base. High power slip ring has successful application in spiral CT. The monitor signal of the jig unit can be acquired by wireless communication or serial infrared communication.

As shown in FIG. 5-FIG. 10, the slurry particle grinding apparatus 1400 includes a grinding zone, wherein said grinding zone includes an annular grinding chamber 141, at least one jet orifice 142 located on the chamber's outer wall. The jet orifice's center line 1421 is either tangential or oblique to the chamber's center line 1411 (tangential in this example). High pressure nozzle 1422 is installed in the orifice. Each jet orifice 142 and its inside high pressure nozzle 1422 combine as a nozzle set. There can be several nozzle sets, such as 2-20 sets. 6 sets are used in this example. These nozzle sets are uniformly distributed along the circumference of the chamber's outer wall. The grinding zone also includes a discharge outlet 143 located on the inner side of the grinding chamber 141 and connected with the chamber. The annulus means a geometry formed by rotating an area 360 degrees along an axis line which is on the plane of the area and doesn't intersect the area. This axis line is called the axis of the annulus. The center line of the grinding chamber means the circumference formed by rotating the geometric center point or near center point of the cross sectional area of the grinding chamber 360 degrees along the axis of the grinding chamber.

Grinding chamber 141 is made of high strength hard material such as carbide ahoy, or compound material of high pressure bearable and high hardness material such as nylon lining in high hardness and wear resisting material such as ceramics, corundum or artificial diamond. To enhance the grinding effect, there are several fins 1412 on the outer, top and bottom walls of the grinding chamber. Fins 1412 are distributed in the following pattern: fins are short and sparsely distributed on the chamber's outer wall; fins are tall and densely distributed on the outer side of top and bottom of the chamber; fins are short and densely distributed on the inner side of top and bottom of the chamber. The shape of fins can be conical, rectangular, cylindrical or combination of them. Cylindrical fins are used in this example.

The cross sectional area of the grinding chamber 141 can be circular, elliptical, drop shape with tip pointing inside or drop shape with long tail. The long tail drop shape is the best fit for the mechanism of slurry deceleration and discharge in the grinding chamber. The long tail drop shape is used in this example.

The mechanism of this grinding apparatus is described as follows. First, the reciprocating pump increases the pressure of raw slurry with particles grinded below the prescribed size to 10-40 Mpa. Then the slurry is injected at a high speed into the grinding chamber by the high pressure nozzle. Solid particles within the slurry are grinded due to strong shear, friction and cavitation effects generated in the injection process. Initially grinded slurry flows at a high speed and in turbulence form in the grinding chamber. Slurry particles are further grinded due to strong collision, friction and shear generated between particles and fins and between particles in the slurry itself. Edges of slurry particles are abraded so that the shape of particles becomes to be spherical. This could improve the fluidity of the slurry. Kinetic energy decreases as slurry rotates from the outer side to the inner side of the chamber. The slurry velocity decreases and finally discharges at the discharge outlet.

If the size of slurry particles doesn't meet the requirement after one grinding process, discharged slurry goes back to the pump and repeats the above grinding process.

Material slurry is pumped to inject through nozzle into an annular grinding chamber tangentially or obliquely. Material rotates with high turbulence in the grinding chamber. Then it slows down and discharges at the discharge outlet. Cross sectional area of both the grinding chamber and the discharge outlet should be much larger than cross sectional area of all nozzles, such as 10000 times and 200 times larger. This could ensure that material in the grinding chamber has low enough pressure to result in large pressure drop along the nozzle. So the velocity of material can be adequately reduced in the grinding chamber. There are strong shear and cavitation effects when material particles are injected through nozzle. In the grinding chamber, there are strong collision, friction and shear between material particles and chamber walls, and between particles in the material itself. All those effects can lead to continuous grinding of material particles and forming sphere particles. This apparatus can produce finer particles and slurry with better fluidity at the same concentration and has lower energy consumption when used for the fine grinding of coal-based slurry fuel.

Although the object, structure and effect of the present invention have been further described with reference to the preferred embodiments thereof, the skilled persons in the art should realize that these examples are just used for illustration purpose and not to limit the invention itself. It should be apparent that variants may be embodied as a part of the present invention and fall into the scope of the pending claims of the present invention. 

1-10. (canceled)
 11. A method of preparing ultra-low ash coal-water slurry comprising the following steps: A) wet or dry grinding of coal to a size below the upper size limit and then making a proper concentration slurry; B) screening coal made in step A by using the slurry particle separation device and separating them by density into tailings, middlings and clean coal, adjusting the fraction of clean coal production according to the ash content standard of final product and then producing the clean coal slurry and tailings slurry; C) wet grinding of middlings in step B; D) reprocessing the product in step C by step B.
 12. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said upper size limit is below 0.2 mm.
 13. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said upper size limit is below 0.01 mm.
 14. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said upper size limit is below 0.001 mm.
 15. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said ash content of final product is no higher than 2%.
 16. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said ash content of final product is no higher than 1%.
 17. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said ash content of final product is no higher than 0.5%.
 18. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein wet grinding is performed using a slurry particle grinding apparatus, the slurry particle grinding apparatus includes a grinding zone, the grinding zone includes an annular grinding chamber, at least one set of nozzle assembly composed of jet orifices and high pressure nozzles, and discharge outlet, the jet orifices are located on the outer wall of the grinding chamber and their center lines are either tangential or oblique to the center line of the grinding chamber, high pressure nozzles are installed in the jet orifices, the discharge outlet is located on the inner wall of the grinding chamber and is communicated with the grinding chamber, cross sectional area of the grinding chamber or the discharge outlet is much larger than the total cross sectional areas of all high pressure nozzles.
 19. The method of preparing ultra-low ash coal-water slurry as recited in claim 11, wherein said slurry particle separation device is a centrifugal jig, the centrifugal jig comprises a rotating device, at least one jig unit and a product receiver, the rotating device includes a rotor and a base, the rotor is power-driven to rotate around a vertical shaft, the jig unit includes a hutch chamber, a screen residue chamber, a screen, a feeding device, a discharging device and a pulsator, the jig unit is installed on the rotor at an inclined angle along the movement of the feed, the hutch chamber is located far away from the vertical shaft to separate particles with different density in the slurry, the product receiver is used to collect the separated materials discharged from the rotating jig unit, separated materials are discharged from an outlet of the screen residue chamber.
 20. A method of preparing ultra-low ash coal-water slurry comprising the following steps: A) wet grinding of coal to a size below the upper size limit and then making a proper concentration slurry; B) screening coal made in step A by using the slurry particle separation device and separating them by density into tailings, middlings and clean coal, adjusting the fraction of clean coal production according to the ash content standard of final product and then producing the clean coal slurry and tailings slurry; C) reprocessing the product in step B by step A.
 21. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said upper size limit is below 0.2 mm.
 22. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said upper size limit is below 0.01 mm.
 23. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said upper size limit is below 0.001 mm.
 24. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said ash content of final product is no higher than 2%.
 25. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said ash content of final product is no higher than 1%.
 26. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said ash content of final product is no higher than 0.5%.
 27. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein wet grinding is performed using a slurry particle grinding apparatus, the slurry particle grinding apparatus includes a grinding zone, the grinding zone includes an annular grinding chamber, at least one set of nozzle assembly composed of jet orifices and high pressure nozzles, and discharge outlet, the jet orifices are located on the outer wall of the grinding chamber and their center lines are either tangential or oblique to the center line of the grinding chamber, high pressure nozzles are installed in the jet orifices, the discharge outlet is located on the inner wall of the grinding chamber and is communicated with the grinding chamber, cross sectional area of the grinding chamber or the discharge outlet is much larger than the total cross sectional areas of all high pressure nozzles.
 28. The method of preparing ultra-low ash coal-water slurry as recited in claim 20, wherein said slurry particle separation device is a centrifugal jig, the centrifugal jig comprises a rotating device, at least one jig unit and a product receiver, the rotating device includes a rotor and a base, the rotor is power-driven to rotate around a vertical shaft, the jig unit includes a hutch chamber, a screen residue chamber, a screen, a feeding device, a discharging device and a pulsator, the jig unit is installed on the rotor at an inclined angle along the movement of the feed, the hutch chamber is located far away from the vertical shaft to separate particles with different density in the slurry, the product receiver is used to collect the separated materials discharged from the rotating jig unit, separated materials are discharged from an outlet of the screen residue chamber. 